The chapter outlines some features of opioid pharmacology, and then details the chemistry, analytical methodology, metabolism, body fluid concentration and influence of pathophysiology on drug metabolism and excretion of morphine. The chapter considers the structure-activity relationships of the morphine molecule in some detail, because of the implications that the active metabolite morphine-6-glucuronide has for clinical practice. The emphasis is the clinical importance of features of the drug's metabolism, toxic or active metabolites, and the practical influence of pathophysiology.

The fact that most patients do achieve at least some degree of pain relief from opioids is a tribute to the efficacy of the drugs. The choice of drug and the prescription regimens remain largely empirical, because there still large gaps in knowledge of the clinical pharmacology of these drugs. These gaps are a function of the variety of chemical structures among opioids and the fact that they are often used in patients whose renal or hepatic function is compromised. Above all most of these drugs are potent, so that doses and concentrations in body fluids are low. Detailed knowledge requires specific and sensitive analytical methods for measuring both parent drugs and metabolites.

Identification of a toxic metabolite of pethidine and an active metabolite of morphine (the two most commonly used opioids) is important for both doctor and patient. It shows the importance of knowledge of both metabolism and excretion for rational prescribing.

The structure of this chapter is that some of the problem areas in opioid clinical pharmacology are discussed first, followed by the metabolic pharmacology of morphine. A previous review [MOORE et al, 1987] covered the chemistry, pharmacology of metabolites, analysis, metabolites in body fluids, enzymology and pathophysiology) of other drugs in the morphine family, buprenorphine, pethidine, methadone and fentanyl (Table 1).

Much of our prescribing of opioids is based on opinion rather than on evidence. In part this is because opioid use in chronic non-cancer pain is an orphan area, in part because trial design and conduct is not easy in this patient group [MCQUAY & MOORE, 1994], and in part it is historic - many opioids are old drugs, and the registration trials required for new drugs have therefore not been done. It is remarkable how little new evidence on oral and intramuscular opioid use has emerged since earlier reviews [MCQUAY, 1989; NAGLE & MCQUAY, 1990; MCQUAY, 1991]. An added complication is that there are many routes by which these drugs can be given. The fact "that it can be done" very often pre-empts the more important question "should it be done?". Enthusiasts for the new route carry it into practice without adequate comparison of risk and benefit with 'established' routes. Again these arguments have been well rehearsed [MCQUAY, 1990]. Much of the 'new' evidence for spinal or transdermal opioids does not answer the real clinical questions.

Just how effective are opioids in managing chronic pain, cancer or non-cancer? The usual claim from audits of the World Health Organisation guidelines for oral opioids in cancer pain is that two-thirds of patients achieve good or moderate pain relief. Why does the pain of the other one-third of patients respond poorly to opioids? The commonest explanation of pain which is poorly responsive is that it is because the pain is neuropathic in character.

There are two extreme positions on opioid responsiveness or sensitivity. One suggests that opioid sensitivity is a relative phenomenon, and therefore that any pain can be controlled by opioids provided that there is an adequate dose escalation and control of adverse effects [PORTENOY et al, 1990]. The other extreme insists that some pains are intrinsically insensitive to opioids and that this insensitivity can be predicted from the clinical characteristics of the pain [ARNER & MEYERSON, 1988]. Nociceptive pain is thought to be sensitive to opioids while neuropathic pain is regarded as insensitive. If neuropathic pain shows an analgesic response with opioid then this has been attributed to mood improvement rather than to a direct effect on pain pathways [KUPERS et al, 1991].

Both extremes of this controversy are supported by a very small number of controlled trials each of which has methodological limitations. These studies have used either single doses [KUPERS et al, 1991; TASKER et al, 1983], infusions of different opioids [ARNER & MEYERSON, 1988; PORTENOY et al, 1990] or have measured pain without simultaneous assessment of adverse effects [ARNER & MEYERSON, 1988; KUPERS et al, 1991; TASKER et al, 1983; ROWBOTHAM et al, 1991]. The flaw with studies which use a single (fixed) dose or infusion rate is that they may underestimate responses in patients with previous opioid exposure. These patients may need more opioid to achieve analgesic effect than the opioid naive.

Using patient-controlled analgesia (PCA) with simultaneous nurse observer measurement of analgesia and adverse effects we gave two concentrations of morphine in a double-blind randomised cross-over fashion and compared the clinical responses produced by both concentrations of morphine [JADAD et al, 1992].

The results did not support the assumption that neuropathic pains are always opioid insensitive. Half of the pains judged as neuropathic achieved a good response. Nociceptive pains collectively showed a better analgesic response, because all of them achieved a good response in at least one of the sessions. No nociceptive pain had a poor response in this study.

It had been suggested that the analgesic response of neuropathic pains to opioids can be explained by the changes in mood induced by the opioids [KUPERS et al, 1991]. When the results of patients with consistent responses were compared, changes in mood reflected changes in pain intensity and relief regardless of the clinical character of the pain, nociceptive or neuropathic. Mood improved when pain intensity decreased or pain relief increased. No patient had a change in mood in the absence of a change in pain intensity or pain relief, and patients with nociceptive pains in fact showed a greater change in mood than those with neuropathic pain. Therefore, the theory that relief of neuropathic pains by opioids is due to changes in mood was not supported by our findings.

Clinicians argue that tolerance to opioids, if it occurs, is driven by disease rather than by pharmacological tolerance. The first problem is that tolerance is used by some to mean any increase in dose, whereas others use it in the more technical sense of an increased dose required to produce the same effect.

It is ingenuous to argue that opioid tolerance does not occur in man - fleeting glimpses have been seen which echo the solid findings of both acute and chronic opioid tolerance in animal models [COLPAERT et al, 1980]. The classic Houde experiments showed chronic tolerance when patients' analgesic response to a test dose was measured before and after chronic dosing [HOUDE, 1985; HOUDE et al, 1966]. The pragmatic issues are whether the dose escalation required by some patients, but which produces difficult adverse effects, could be avoided (safely) by blocking a tolerance-induced need for dose escalation, or (more simply) by changing opioid or indeed route of administration. The academic question is why some patients do not require dose escalation, but continue to maintain good relief on the same dose over many months.

Clinical pain management has emphasised a difference between the clinical and the laboratory pharmacology of opiates. It is as though there is one opiate pharmacology when the opiate is used to counteract pain, and another when it is not.

The respiratory depression which haunts prescribers in acute pain management is seen readily in studies of volunteers who are not in pain. For patients with opiate-sensitive pain, given appropriate doses of opiate, respiratory depression is minimal. The balance between pain and opiate respiratory effects is seen clearly in chronic pain. Patients maintained on oral morphine, with no clinical respiratory depression, and who then receive successful nerve blocks, must have their morphine dose reduced. Failure to reduce the dose will result in respiratory depression [HANKS et al, 1981; MCQUAY, 1988]. One explanation is that the respiratory centre receives nociceptive input [ARITA et al, 1988]. Presence of this input counterbalances any respiratory depressant effect of the opiate. Absence of this input, because of the successful nerve block, leaves the respiratory depressant effect of the opiate unopposed. This has been shown beautifully in volunteers [BORGBJERG et al, 1996].

The clinical message is that opiates need to be titrated against pain. Doses higher than necessary for the relief of pain run the risk of respiratory depression. Prophylactic use of opiates, infusion without regard to pain experienced, doses greater than those required for analgesia (as in deliberate ITU use to facilitate ventilation of a patient), use for purposes other than analgesia (e.g. sedation), or use in non-nociceptive pain, all therefore carry potential risk. Concern about respiratory depression should not inhibit the appropriate use of opioids to provide analgesia when the pain may reasonably be thought to be opiate sensitive. A postoperative patient still complaining of pain when the previous dose can be assumed to have been absorbed needs more drug.

Similarly the drug-seeking behaviour synonymous with street addiction is not found in patients after pain relief with opiates, in childbirth, after operations or after myocardial infarction [PORTER & JICK, 1980]. Street addicts are not in pain. The political message is that medical use of opiates does not create street addicts. Medical use may indirectly increase availability to those who are already addicts, but restricting medical use hurts patients.

To make sense of morphine metabolism the structure-activity relationships must be understood. Alterations to the structure change the pharmacological activity and may have important clinical sequelae. The basic principles have been known for some time, and were well summarised in a WHO Bulletin published as long ago as 1955 [BRAENDEN et al, 1955].

The most important positions on the morphine molecule, because of their implications for both activity and morphine metabolism, are the phenolic hydroxyl at position 3, the alcoholic hydroxyl at position 6, and at the nitrogen atom (Figure 1).

Both hydroxyl groups can be converted to ethers or esters (e.g. heroin, diacetylmorphine) and these changes alter clinical effect. Changes on the hydroxyl groups are opposite in direction; additions at the phenolic 3-hydroxyl group reduce pharmacological activity considerably, by perhaps more than 90%. By contrast, modification at the alcoholic 6-hydroxyl position results in an activation of the molecule, with the resulting compound being 2-4 times more potent as an analgesic than morphine after parenteral dosing in standard tests.

These rules are not absolute, however, and some substitutions at the 6-hydroxyl (e.g. conjugation with long aliphatic acids) reduce activity because of steric and other considerations. Short chain fatty acid substitutions (such as 3,6-dibutanoylmorphine) have been used to increase the lipophilicity and potency of morphine [OWEN & NAKATSU, 1984; TASKER AND NAKATSU, 1984].

The tertiary character of the nitrogen atom is crucial for morphine's analgesic activity. Chemical modifications which make the nitrogen quaternary (as with N -oxide) greatly diminish analgesic potency because of reduced penetration into the central nervous system. Changes to the methyl substituent on the nitrogen are also important; replacement of the methyl group with 3-carbon alkyl groups not only reduces the analgesic action, but actually produces compounds which antagonise the actions of morphine, such as nalorphine.

Studies of morphine kinetics and metabolism require adequate methods of analysis. Results from inadequate analytical methods should be interpreted with caution. The kinetic and dynamic differences demonstrated between different species increase the difficulties.

Svensson et al [SVENSSON et al, 1982] developed a high-performance chromatographic procedure (HPLC) which measured morphine and its 3- and 6-glucuronides simultaneously. These analyses were facilitated by the high concentrations found in plasma when patients take large oral morphine doses. Similar results were also described using differential radioimmunoassay [HAND et al, 1987a], where samples were measured with morphine antisera of different specifics. A specific radioimmunoassay for the determination of morphine-6-glucuronide in human plasma has been reported [CHAPMAN et al, 1995].

Morphine glucuronides are formed by enzyme catalysed transfer of glucuronic acid from uridine diphosphoglucuronic acid (UDP); the enzymes responsible are microsomal UDP glucuronyl transferases (UDPGT). This is a series of functionally distinct enzymes found in liver, kidney, intestines and other organs. The products of glucuronidation are excreted by the urine and bile. Whether the glucuronide is excreted by urine or bile depends upon the molecular weight and polarity of the conjugate. Compounds with larger molecular weight (more than 300 Da) and low water solubility are more often excreted in the bile. Morphine glucuronides, being very water soluble, are expected to be excreted in the urine.

It is generally assumed that morphine conjugation occurs primarily in the liver, though the evidence is not compelling. For instance, the Michaelis constant for hepatic glucuronidation in human and animal tissue is of the order of 2 mmol.L -1 [SAWE et al, 1982], which is some tens of thousands of times greater than the usual plasma concentrations of morphine. The implication of this high Michaelis constant is that, at therapeutic concentrations of around 200 nmol.L -1 , the liver microsomal glucuronidation systems would work far too slowly to account for the rates of morphine glucuronidation. As an example of this, rat hepatocytes were able to glucuronidate nalorphine but not morphine, even at high intracellular concentrations [IWAMOTO AND KLAASEN, 1978].

Different morphine conjugates may arise from the actions of different enzymes. When the natural (-) and the synthesised (+) morphine enantiomers were tested for glucuronidation, the (+) enantiomer was preferentially conjugated at the 6-position of the conjugate rather than the 3-position [RANE et al, 1985]. This work serves to emphasise the complexity of morphine metabolism at the sub-cellular (rather than the whole-body) level.

Coughtrie et al [COUGHTRIE et al, 1989] showed that morphine glucuronide formation was influenced by both enantiomer and body 'region'. In rat liver microsomes, natural (-)-morphine formed only the 3-O-glucuronide, whereas the unnatural (+)-morphine formed glucuronides at both the 3-OH and 6-OH positions, with the 6-O-glucuronide being the principal product. In human liver microsomes, both the 3-OH-and 6-OH positions were glucuronidated by each of the enantiomers, the 3-O-glucuronide being the major product with (-)-morphine, and the 6-OH position preferred by the (+)-enantiomer. Two UDP-glucuronosyltransferase isoenzymes were responsible for the glucuronidation of morphine in rat liver. Morphine UDP-glucuronosyltransferase produced glucuronides at both the (-)-3-OH and (+)-6-OH positions, the other formed only the (+)-morphine-3-glucuronide. In human kidney, there was glucuronidation ability at the 3-OH but not the 6-OH position.

Dechelotte et al [DECHELOTTE et al, 1993] compared morphine uptake and biotransformation to M3G and M6G in isolated cells from guinea pig stomach, intestine, colon, and liver. Morphine was glucuronidated to M3G by gastric, intestinal, colonic, and liver cells, and to M6G by all except gastric cells. They found that small and large intestine epithelium, like liver, formed M6G, and that gastric, intestinal, and colonic epithelia inactivated morphine to M3G.

Knodell et al [KNODELL et al, 1982] ligated the inferior vena cava above the entrance to the hepatic veins, and reduced the hepatic blood flow to less than 50% of controls; morphine clearance was unaltered, and the conclusion was that there were extra-hepatic sites for morphine metabolism. Similar conclusions have been made in man, where the disposition and elimination of indocyanin green and morphine were studied in healthy controls and cirrhotic patients [PATWARDHAN et al, 1981]. There was a significant decrease in indocyanin green clearance, but no alteration in morphine kinetics, again with the suggestion of some extrahepatic site of glucuronidation. This is unlikely to be the intestine, as drugs (like morphine) with low lipophilicity are not subject to extensive gut wall glucuronidation [RANCE AND SHILLINGFORD, 1977]. The inability of the gut to metabolise morphine is supported by in-vivo data [KNODELL et al, 1982].

There is, unfortunately, no clear evidence for an alternative organ of metabolism. Rabbit kidney tubules are able to metabolise morphine to glucuronides at the same sort of concentrations found in plasma in vivo [SCHALI AND ROCH-RAMEL, 1982], and the perfused rat kidney can actively excrete morphine [RATCLIFFE et al, 1985]. These data, however, do not substantiate the idea of significant renal glucuronidation in man.

Milne et al [MILNE et al, 1993] used sheep to study the regional formation and extraction of M3G and M6G. There was significant extraction of morphine by the liver and kidney, net extraction of M3G and M6G by the kidney, and net formation of M3G by the gut. In a subsequent paper [MILNE et al, 1995] they infused morphine or M3G into sheep and calculated regional net extraction ratios and total and regional clearances. They found prolonged elimination of M3G formed in situ from morphine compared with after M3G infusion. M3G was not converted back into morphine or M6G.

Mazoit et al [MAZOIT et al, 1990] measured the hepatic extraction ratio of morphine directly in six patients having radiological procedures. The hepatic extraction ratio was 0.65 +/- 0.11. No concentration gradient was observed between the artery and the superior mesenteric vein, showing that no gut wall metabolism of morphine occurred. Total body clearance was 38% greater than the hepatic clearance, and they concluded that the extrahepatic extraintestinal clearance of morphine probably occurred through the kidney.

Van Crugten et al [VAN CRUGTEN et al, 1991] studied the renal handling of morphine, M3G and M6G in an isolated perfused rat kidney. They found renal handling of morphine to be a complex combination of glomerular filtration, active tubular secretion, and possibly active reabsorption, with the glucuronide metabolites, larger and less lipophilic than morphine, undergoing net tubular reabsorption.

The ability of M6G to penetrate the blood-brain barrier unchanged was confirmed using radioactively labelled M6G [YOSHIMURA et al, 1973]. The analgesic activity of M6G did not appear to be due to hydrolysis of the conjugate in the brain or elsewhere; only conjugated morphine was found in rat brain after intraperitoneal M6G injections [SHIMOMURA et al, 1971]. Evidence of transformation of M6G to morphine in brain tissue has been conflicting. Wahlstrom et al [WAHLSTROM et al, 1988] showed ability of CNS to metabolise M6G to morphine. Sandouk et al [SANDOUK et al, 1991] found, after intracerebroventricular administration of morphine in four cancer patients, that brain was able to metabolise morphine to M 3-G and M 6-G.

In man more than 90% of an administered dose of morphine is excreted in the urine. Only about 10% is unchanged morphine, and M3G is the major metabolite (Table 1). M6G was thought to be a minor metabolite, with less than 1% of the dose being in this form in post-addict males on high doses; more recent studies, using plasma samples, suggest that this figure is much too low [HAND et al, 1987a]. Using differential radioimmunoassay, the amounts of morphine, and 3- and 6-glucuronide found in the urine of patients on oral morphine therapy were in the mean ratio of 1:20:1.5 [Hand, McQuay, Moore, unpublished observations). The diglucuronide (morphine-3,6-diglucuronide) was also found in urine to a small extent [YEH et al, 1979].

Morphine-3-ethereal sulphate (the major metabolite of morphine in the cat and chicken [MORI et al, 1972]) accounts for perhaps 5% of a dose of morphine in man [YEH et al, 1977]. Ethereal sulphates are formed through the action of hepatic microsomal sulphokinases [BOERNER et al, 1975]. Morphine-6-sulphate, though sought, has not been identified in any species.

Normorphine and normorphine-6-glucuronide have also been found in human urine [YEH et al, 1977]. Normorphine is formed by hepatic microsomal oxidation and can account for about 5% of urinary excretion products of morphine in man [BOERNER et al, 1975].

Minor metabolites of morphine, such as codeine (3- O -methyl morphine) and morphine N -oxide have been identified in the urine of humans taking large doses of morphine chronically [YEH et al, 1977]. They account for minor proportions (less than 1%) of an administered dose of morphine in man.

In plasma, only morphine, M3G and M6G have been identified. M6G was not thought to be present until recent years. Studies using HPLC demonstrated that appreciable levels of M6G were found in the plasma of cancer-pain patients on high oral doses of morphine. The M6G levels were higher than those of morphine itself, and about 10% of the concentration of M3G [SVENSSON et al, 1982].

One study which encapsulates the plasma data [HASSELSTRÖM & SÄWE, 1993], found, after single intravenous 5 mg and oral 20 mg doses of morphine in 7 healthy volunteers, that clearance of morphine to form M3G and M6G was 57.3% and 10.4%, respectively, and renal clearance was 10.9% of total systemic plasma clearance. Twenty percent of a dose remained as unidentified residual clearance. The proportions of the dose found as M6G and M3G were the same by either route. A major finding was a slowly declining phase of morphine and metabolites that was evident both In both plasma and urine the terminal half-lives were long, morphine 15.1 +/- 6.5 h, M3G 11.2 +/- 2.7 h and M6G 12.9 +/- 4.5 h. A greater proportion of morphine and metabolites was excreted during the slowly declining phase after the oral dose than the intravenous dose, which they suggested was due to enterohepatic recycling. The renal clearance of M6G and morphine was exceeded creatinine clearance, which they attributed to an active secretion process.

The time course for the plasma concentrations (measured by using differential radioimmunoassay) of morphine, M3G and M6G after a single oral dose of 10 mg morphine is shown in Figure 2.

Barjavel et al [BARJAVEL et al, 1994] used transcortical microdialysis after subcutaneous morphine, M3G and M6G in rats. Maximum brain opiate concentrations were reached at the same time, 0.75 h. Penetration and elimination rates in the extracellular space of the rat brain cortex for the hydrophilic metabolites were similar to those of morphine. They concluded that in spite of their structural differences the glucuronide metabolites were capable of crossing the blood-brain-barrier at the same rate as morphine, but in greater quantity.

Stain et al [STAIN et al, 1995] found that subcutaneous M6G at the same doses as morphine produced a greater degree of analgesia with longer duration of action on behavioural tests. Concentrations of morphine and M6G in brain extracellular fluid were measured using microdialysis. They concluded that M6G was much more potent than morphine in the rat and attributed the difference to the higher levels of M6G in plasma and brain extracellular fluid.

There is also the prospect of M6G working at a different receptor. Rossi et al [ROSSI et al, 1995] used antisense oligodeoxynucleotides directed against distinct Gi alpha subunits of the morphine receptor to distinguish between morphine and M6G analgesia. The insensitivity of M6G towards the MOR-1 antisense probe and differential sensitivity towards G-protein alpha subunit antisense oligodeoxynucleotides led them to believe that M6G acts through a different opioid receptor than morphine.

M3G has no analgesic activity [SCHULZ AND GOLDSTEIN, 1972], but it does have central nervous system (CNS) stimulatory effects not mediated through opioid receptors [WOOLF, 1981; YAKSH et al, 1986; YAKSH & HARTY, 1988]. Reports of antagonism of morphine analgesia by M3G [SMITH et al, 1990; GONG et al, 1992] are best interpreted as functional, because there is good evidence that there is no direct pharmacological antagonism [HEWETT et al, 1993; SUZUKI et al, 1993]. This is not surprising given the fact that M3G does not bind to opioid receptors.

The more recent evidence of M6G action on opioid receptors was anticipated by the observations that it was antagonised by nalorphine and demonstrated cross tolerance with morphine [SHIMOMURA et al, 1971). M6G, unlike M3G, was 3-4 times more potent than morphine as an analgesic after subcutaneous injection in mice, and 45 times more potent after intracerebroventricular injection [SHIMOMURA et al, 1971]. The difference in the potency ratio was attributed to slower entry of the glucuronide into the CNS compared with morphine [SHIMOMURA et al, 1971], and nalorphine-6-sulphate had been shown to be a more potent antagonist than nalorphine itself [OGURI, 1980]. This greater analgesic potency has been confirmed, with 10 to 20 times greater intrathecal potency of M6G in the rat compared with morphine [PASTERNAK et al, 1987; SULLIVAN et al, 1989].

The greater potency of M6G compared with morphine has important clinical implications [OSBORNE et al, 1990; MCQUAY et al, 1990]. It has long been known quantitatively to be an important metabolite [BOERNER et al, 1975; SHIMOMURA et al, 1971]. Many studies have now investigated the ratios of parent to metabolite plasma concentrations (Table 2).

Table 2 and Figures 3 and 4 show the ratios of metabolites to morphine in plasma after single doses of morphine, and the ratios of CSF:plasma concentrations for the two metabolites. Despite a variety of assays used and disparate study designs, a pattern does emerge. After single oral doses the median M6G:morphine plasma concentration ratio was 5.4 (range 0.96-11, n=11), and for M3G:morphine 25.0 (range 9.9-56, n=11) (Table 4).

The corresponding ratios after single intravenous doses were about six times lower at 0.6 (range 0.29 - 2.0) for M6G:morphine plasma concentration ratio and 6.1 (2.8 - 11.1) for M3G:morphine (Tables 2 and 4). The difference reflects the first-pass metabolism which applies to the oral route but not the intravenous.

These are similar to those found after single oral doses. This implies that there is little difference in metabolism between the single and multiple dosing contexts. It also means that studies of this phenomenon done after either single or multiple doses will be equally valid extrapolated to the other situation. The M6G:morphine plasma ratios of 3.6 and 5 lead straight back into the argument about how much M6G contributes to the total analgesic effect of a dose of morphine (Table 6) [HAND et al, 1987b; MCQUAY et al, 1987; OSBORNE et al, 1988].

The crucial observation for morphine in man is whether the active metabolite morphine-6-glucuronide appears in the central nervous system to interact with opioid receptors and thus produce analgesia. In Table 2 the csf:plasma ratios of M6G and M3G after single doses are shown, and after multiple doses in Table 3. Summmarised (Table 5) there is clear evidence that M6G does indeed penetrate into CSF from plasma.

There is some hint that M6G penetrates to a greater extent than M3G, perhaps by a multiple of between 2 and 4. Looking at the difference in CSF:plasma ratios between single and multiple doses, the small amount of data shows higher values for both M6G and M3G in CSF after multiple doses. The values for multiple epidural and intrathecal dosing are, not surprisingly, much higher.

Given the greater potency the puzzle then is how much does M6G contribute to the total analgesia resulting from a dose of morphine? Simple calculation suggests that M6G may contribute substantially (Table 6). If the relative potency of M6G to morphine in the CSF of man is similar to that after intrathecal injection in rodents, i.e., about 10-20 fold greater potency [SULLIVAN et al, 1989], then it may be argued that about 60% of the analgesia from multiple doses of morphine may be due to M6G (Table 6).

Against this several clinical studies have failed to show any relationship between M6G plasma concentrations and analgesia or other opioid effects [SOMOGYI et al, 1993; VAN DONGEN et al, 1994], or indeed between M6G:morphine plasma concentration ratio and effect, although others have been more successful [PORTENOY et al, 1992; FAURA et al, 1996], and some have approached this through modelling techniques [WESTERLING et al, 1995]. There are still no adequate randomised double-blind studies of the analgesic and other effects of M6G given on its own.

Hanna et al [HANNA et al, 1990] compared the analgesic efficacy of intrathecal M6G 500 ug with morphine 500 ug in a single-blind crossover study of three patients with chronic cancer pain. The mean (SD) requirement for patient controlled analgesia with pethidine was 393.3 (227.4) mg/24 h during the morphine part of the trial and 226.7 (113.6) mg/24 h with M6G.

Peat et al, [PEAT et al, 1991] studied the respiratory responses to intravenous morphine (0.12 mg/kg), M6G (0.03 mg/kg) and placebo in 6 volunteers, using a single blind randomised crossover design. Five volunteers also had M6G 0.06 mg/kg. After placebo or M6G (at both doses) there was no change in end-tidal CO2 whilst the subjects were breathing air. After morphine there was a significant rise. Morphine reduced the ventilatory response to 5.5% CO2 significantly at all times tested. M6G (at both doses) reduced the response to CO2 at 20 and 40 min after administration, but to a significantly lower extent than morphine.

Osborne et al [OSBORNE et al, 1992] compared the cardio-respiratory and analgesic effects of four different dose levels (0.5, 1, 2, and 4 mg /70 kg) of intravenous M6G in an open study of 20 cancer patients with pain. M6G exerted a 'useful' analgesic effect in 17/19 patients for periods ranging between 2 and 24 h. No correlation was observed between dose or plasma M6G concentrations, and duration or degree of analgesia. No clinically significant changes in cardio-respiratory parameters were observed. No patients reported sedation or euphoria. Nausea and vomiting were 'notably absent' in all cases.

These preliminary studies do suggest an analgesic effect of M6G, but the claim that M6G has less respiratory depressant potential than morphine will have to be addressed at equianalgesic dosing, and it is not clear that this has been achieved.

Traditional teaching is that use of morphine in patients with liver disease may result in excessive sedation and precipitate hepatic encephalopathy [LAIDLAW et al, 1961; TWYCROSS AND LACK, 1983]. Two studies of morphine kinetics in patients with cirrhosis [PATWARDHAN et al, 1981; HASSELSTROM et al, 1986] have shown only very minor alterations of morphine kinetics in cirrhotic patients.

Subsequent work, however, has shown kinetic change with liver disease. Hasselstrom et al [HASSELSTROM et al, 1990] studied oral and intravenous kinetics of morphine in seven cirrhotic patients with a history of encephalopathy. Morphine plasma clearance was significantly lower, its terminal elimination half-life longer and its oral bioavailability greater in the cirrhotic patients compared with patients with normal liver function. Plasma M3G:morphine ratios were significantly lower in the cirrhotic patients after oral, but not after intravenous, doses. Mazoit et al [MAZOIT et al, 1987] compared plasma morphine concentrations in six volunteers with those in eight cirrhotic patients. Cirrhotics had significantly longer morphine terminal half-life, attributed to lower total body clearance.

D'Honneur et al [D'HONNEUR et al, 1994] compared plasma and CSF concentrations of morphine glucuronides in patients with normal renal function and those with renal failure. Plasma concentrations of glucuronides were significantly higher. CSF concentrations of M6G and M3G continued to rise over at least 24 hours. At 24 hours CSF M6G concentrations were fifteen times greater in patients with renal failure than in those with normal renal function.

Hanna et al [HANNA et al, 1993] gave 30 ug/kg M6G to 12 patients with chronic renal failure (dialysis-dependent) and 6 with good renal function after renal transplantation. The M6G elimination half-life was significantly shorter, and the clearance greater, for the transplanted group compared with the dialysed and non-dialysed groups.

Osborne et al [OSBORNE et al, 1993] compared the pharmacokinetics of morphine and its glucuronide metabolites in three groups of patients with kidney failure (nondialyzed, receiving dialysis, and transplantation) with a group of normal healthy volunteers. Patients with kidney failure had a significantly increased morphine area under the curve (AUC) compared with control subjects. There was also an increase in M3G and M6G that was several times greater than the increase in morphine AUC. This metabolite accumulation was reversed by kidney transplantation.

Peterson et al [PETERSON et al, 1990] found in 21 patients on oral or subcutaneous morphine that plasma concentrations of M3G and M6G, when divided by the morphine concentration, were significantly related to the calculated creatinine clearance of the patient.

Sear et al [SEAR et al, 1989a] compared intravenous morphine 10 mg kinetics in nine patients with end-stage renal failure with five healthy anaesthetised patients. There were no differences between the two groups for morphine elimination half-life or clearance. Peak concentrations of M3G and M6G were significantly greater in the renal transplant patients, as were the AUCs (0-24 h).

Somogyi et al [SOMOGYI et al, 1993] studied 11 cancer patients on long term oral morphine. They could not detect a relationship between the renal clearance of morphine, M3G and M6G, and that of creatinine. Renal tubular handling of all 3 opioids varied widely between patients, and there was evidence of either net renal tubular secretion or reabsorption.

This renal work highlights the role of the kidney in removing the major morphine metabolites. The evidence would seem to be that once renal function declines to a creatinine clearance of 50 ml min -1 or below, then accumulation of morphine metabolites, and especially M6G [BODD et al, 1990] will become significant. Clinical problems should arise only if fixed dose schedules are used at too high an initial dose level.

Earlier reports of interactions between cimetidine and morphine were rebutted [MOJAVERIAN et al, 1982]. Wahlstrom et al [WAHLSTROM et al, 1994] studied the interactions of tricyclic antidepressants with morphine glucuronidation. All tricylics studied inhibited morphine glucuronidation, nortriptyline in non-competitive and amitriptyline and clomipramine in competitive or mixed manner. Inhibition occurred at a concentration ratio of tricyclic to morphine close to that seen in patients on treatment.

Much of the detail of morphine metabolism and excretion is still to be worked out. The clinical implication thus far is that the accumulation of the active metabolite M6G in renal dysfunction may produce both analgesia and unwanted effects. Fixed dose schedules with high initial doses should be avoided if renal problems are suspected. The precise contribution of M6G to the 'total' effect of a dose of morphine remains a puzzle.